1. Field of the Invention
The present invention relates to a method of heat-treating a semiconductor wafer and, more specifically, to improvements in a method of heat-treating a semiconductor wafer, using a gas capable of absorbing infrared radiation in a specific infrared region as a process gas.
2. Description of the Prior Art
A lamp type heating device provided with a tungsten lamp or a halogen lamp as a light source for the rapid infrared heating of a semiconductor wafer is used for manufacturing semiconductor devices. Generally, the temperature of a wafer is measured by a pyrometer when heating the wafer with such a heating device. Most pyrometers use a long monitoring wavelength in the range of 4 to 5 .mu.m to measure temperatures in a low temperature range of 300.degree. to 400.degree. C.
The enhancement of the reliability of a very thin SiO.sub.2 film of a thickness in the range of 5 to nm is essential to form VLSIs and ULSIs of a high degree of integration. Such a very thin SiO.sub.2 film of high reliability is important particularly in forming floating gate memories, such as EPROMs, EEPROMs and flash memories. A process for the thermal nitriding of a SiO.sub.2 film in a NH.sub.3 atmosphere has been studied to enhance the reliability of the SiO.sub.2 film. However, when the SiO.sub.2 film is processed in a NH.sub.3 atmosphere for thermal nitriding, the breakdown characteristics of the SiO.sub.2 film are deteriorated greatly. Such a nitrided SiO.sub.2 film is unreliable for long-term use.
To solve such a problem a highly reliable N.sub.2 O-nitrided SiO.sub.2 film having few traps has been proposed in, for example, H. Fukuda, et al., "Highly Reliable Thin Nitrided SiO.sub.2 Films Formed by Rapid Thermal Processing in an N.sub.2 O Ambient", Extended Abstracts of 1990 International Conference on Solid State Devices and Materials, pp.159, 1990. This N.sub.2 O-nitrided SiO.sub.2 thin film can be obtained by subjecting a SiO.sub.2 thin film to, for example, RTA (rapid thermal annealing) in a nitrogen monoxide (N.sub.2 O) gas atmosphere.
When N.sub.2 O gas is employed as a process gas, it is impossible to measure the temperature of the wafer accurately, because N.sub.2 O gas has an infrared absorption peak in the wavelength range of 4.5 to 5.0 .mu.m as shown in FIG. 1, which is a quotation from H.L. Hackfold, "Sekigaisen Kogaku", Kindai Kagaku She, Oct. 1, 1963, pp. 54 and 66.
FIG. 2 is a graph showing the variation of the output of the pyrometer with time when a wafer was heated at a constant temperature in a heating chamber, and N.sub.2 gas and N.sub.2 O gas were supplied alternately into the heating chamber at a cycle time of 20 sec. The monitoring wavelength of the pyrometer was 4.7 .mu.m. As is obvious from FIG. 2, the output of the pyrometer is stabilized on a level corresponding to abut 1050.degree. C. while N.sub.2 gas is supplied into the heating chamber, whereas the output of the pyrometer is reduced to levels corresponding respectively to about 780.degree. C. and 720.degree. C., which are lower than the temperature corresponding to the level of the output of the pyrometer when N.sub.2 gas is supplied into the heating chamber by about 300.degree. C., while N.sub.2 O gas is supplied into the chamber, due to the absorption of infrared radiation emitted by the wafer by the N.sub.2 O gas. Whereas the outputs of the pyrometer in the first and second N.sub.2 gas supply cycles are substantially equal to each other, the difference between the output of the pyrometer in the first N.sub.2 O gas supply cycle indicating about 720.degree. C. and that of the same in the second N.sub.2 O gas supply cycle indicating about 780.degree. C. is about 60.degree. C., which is inferred to be due to the variation of the temperature of the N.sub.2 O gas.
It is obvious from FIG. 2 that it is quite difficult to measure the temperature of the wafer by the pyrometer using a monitoring wavelength of 4.7 .mu.m when N.sub.2 O gas is used as a process gas, because infrared radiation emitted by the wafer are absorbed by the N.sub.2 O gas and the absorption of the infrared radiation by the N.sub.2 O gas is dependent on the temperature of N.sub.2 O gas.
As shown in FIG. 1, similar difficulty occurs when a process gas of triatomic molecules having an infrared absorption band corresponding to the infrared region of 4.5 to 5.0 .mu.m, such as steam, carbon dioxide gas (CO.sub.2 gas) or ozone, is used instead of N.sub.2 O gas.